a. Field of the Invention
Broadly speaking, this invention relates to equalization. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for adaptively amplitude equalizing a communications channel which is subject to frequency-selective fading.
B. Discussion of the Prior Art
As is well known, microwave radio channels frequently experience deep fading in rain or fog. Service is maintained over such channels by providing sufficient power at the transmitter to ensure an adequate signal-to-noise ratio at the receiver even in the presence of a deep fade, and by providing the receiver with automatic gain control.
In addition to fades caused by rain or fog, it has been discovered that unusual atmospheric conditions may support microwave propagation over two or more distinct paths between any two, line-of-sight radio antennas in the microwave system. The various signal paths typically differ in their propagation delay, thus permitting both constructive and destructive interference at the receiving antenna. When the relative delay is significant with respect to the period of the radio frequency signal, the interference can be quite selective, with deep nulls in parts of the band and smaller variations at adjacent frequencies.
The variation in the received power is called fading, and the variation in fading as a function of frequency is known as selective fading. During non-selective fading, for example during heavy fog, the signal level across the microwave channel remains flat and simply drops in level. When the channel is experiencing selective fading, however, the channel characteristic contains one or more minima which can be quite sharp. The most important features of a selective fade are its depth and the variation of the response across the channel. The depth of a typical selective fade may be as much as 40 dB while the rate at which the fade occurs may be as high as 100 dB/second.
Heretofore, virtually all microwave systems employed frequency modulation, which is relatively insensitive to both selective and non-selective fading. Recently, however, there has been considerable interest expressed in converting existing F.M. systems to the use of amplitude modulation, particularly suppressed-carrier, single-sideband modulation. In many respects amplitude modulation promises to be more efficient than frequency modulation; unfortunately, it is far more sensitive to the effects of frequency-selective fading than is frequency modulation.
The problem of selective fading has, of course, been known at lower frequencies for many years. For example, U.S. Pat. No. 2,054,657, which issued Sept. 15, 1936 to H. Mayer, discusses a circuit which may be used to compensate for the effects of selective fading in a short-wave radio receiver. In the circuit discussed by Mayer, the frequency band to be regulated is divided into a number of sub-bands, each of which is separately regulated, for example, by means of a pilot tone transmitted within each of the sub-bands. For satisfactory regulation, it is necessary that the sub-bands be closely spaced, i.e., that they overlap, and this is typical of prior art "multi-bump" amplitude equalizers.
U.S. Pat. No. 2,719,270, which issued Sept. 27, l955 to R. W. Ketchledge, discloses a somewhat more sophisticated equalizer for use with coaxial cable carrier systems. In the Ketchledge patent, three, tandem-connected adjustable equalizers having overlapping but differently shaped frequency characteristics are controlled by three pilot tones respectively transmitted at the edges and center of the message band. After passage through the equalizer, the pilot tones are filtered from the message band, amplified, detected, and compared with a reference voltage. An analogue computer connected to the three comparison circuits then determines the changes which must be made to the shapes of the three bump equalizers to effect the desired overall equalizer adjustment.
Each of the three pilot tone feedback circuits in the Ketchledge equalizer contains amplification. This fact, plus the fact that the equalizer sections have overlapping frequency characteristics, lays open the possibility of instability and oscillation in the equalizer. This does not, in fact, occur in the Ketchledge equalizer because it is designed for use in a coaxial cable system where changes in attenuation occur slowly and are of limited amplitude. Thus, the bandwidth of the pilot tone filters and the gain around the three feedback loops can be selected such that the equalizer is stable at all times.
Unfortunately, the circuit disclosed in the Ketchledge patent is not capable of compensating for frequency-selective fading in a microwave system because of the extremely rapid rate at which such fading takes place, as well as its relatively large magnitude. To satisfactorily equalize for frequency-selective fading in the microwave region, it is necessary that the equalizer contain both a considerable amount of negative feedback and wide bandwidth so that it may respond to the rapid changes in signal level. Thus, the simple approach proposed by Mayer and Ketchledge would, in the presence of such feedback, cause oscillations and instability, thus rendering the equalizer inoperative.
The problem of providing adaptive amplitude equalization in a microwave channel subjected to extremely rapid frequency-selective fading has been solved by the instant invention which, in a preferred embodiment, comprises apparatus for adaptively equalizing the amplitude of a communications channel which has impressed thereon a message load of predetermined bandwidth, first and second pilot tones respectively positioned proximate the upper and lower edges of the band and at least a third pilot tone positioned proximate the center of the band. More particularly, the apparatus comprises, in tandem, means for shaping the frequency characteristic of a first portion of the band and means for shaping the frequency characteristic of a second portion of the band, the first and second shaping means being designed such that they have substantially no shaping effect on that portion of the band shaped by the other. The equalizer also includes a variable-gain amplifier connected downstream of the first and second shaping means, means connected to the output of the amplifier for filtering the first, second and third pilot tones from the message load and means connected to the output of the filtering means for detecting the first, second and third pilot tones. The apparatus also includes a source of reference potential and means connected to the reference source and to the detecting means for selectively adjusting the gain of the variable gain amplifier and/or the slope characteristics of the first and second shaping means, in an offsetting manner, as the levels of the first, second and third tones vary, whereby the overall amplitude characteristic of the communications channel remains substantially flat even in the presence of frequency-selective and non-frequency-selective fading in the channel.
The invention and its mode of operation will be more fully understood from the following detailed description when taken with the appended drawings in which: